Progressive multifocal leukoencephalopathy

'\Progressive multifocal leukoencephalitis' (or leukoencephalopathy) is a rare disorder that is characterized by progressive inflammation (-itis) of the white matter (leuko-) of the brain (-encephalo-) at multiple locations (multifocal). The white matter is mostly composed of axons of the nerve cells that are emerging from the outermost parts of the brain (cortex). The myelin sheath covering the axons and increasing their conduction velocities is preferentially affected. It appears to be a virus-induced disease and is most frequently observed in patients with immune deficiency (e.g. AIDS).